Various kinds of silver halide photographic materials have been commercially sold in the market and various methods of processing them for image formation have been known. Such materials are therefore utilized in various fields. The halogen composition of silver halide emulsions constituting these various photographic materials is in many cases, especially in the case of picture-taking photographic materials, a silver iodobromide consisting essentially of silver bromide, since the materials are desired to have a high sensitivity.
On the other hand, in the case of photographic materials for color photographic paper products which are used in the market where a large amount of color prints are desired to be processed and finished in a short period of time for delivery to the consumers and users, a silver bromide or silver chlorobromide which does not substantially contain silver iodide is used in the emulsions because of the necessity of accelerating the speed of development thereof.
Recently, the request for improvement of rapid processability of color photographic papers is increasing more and more, and many studies thereof have been made. For instance, it is known that elevation of the silver chloride content in a silver halide emulsion brings about noticeable improvement of the developability of the resulting emulsion.
However, it is known that a high silver chloride emulsion could hardly have a high sensitivity to give a hard gradation. Additionally, it is also known that the emulsion has the drawback of a large reciprocity law failure. That is, variation of the intensity of light to be applied to the emulsion for exposure causes great fluctuation of sensitivity and gradation of the emulsion.
In order to overcome the above-mentioned drawbacks of such a high silver chloride emulsion, various techniques have been illustrated.
For instance, JP-A-58-95736, JP-A-58-108533, JP-A-60-222844 corresponding to U.S. Pat. No. 4,590,155, JP-A-60-222845 corresponding to U.S. Pat. No. 4,605,610 and JP-A-64-26837 corresponding to U.S. Pat. Nos. 4,820,624 and 4,865,962 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") illustrate various high silver chloride emulsions with various silver bromide-rich regions of various structures which have high sensitivity to give hard images. However, after repeated investigations made by the present inventors, it has been found that high silver chloride emulsions prepared by the illustrated techniques often cause desensitization under pressure, though they are highly sensitive in the absence of pressure. Therefore, the emulsions have a severe drawback for practical use. In addition, it has also been found that the reciprocity law failure of high silver chloride emulsions could not fully be overcome by the illustrated techniques.
In order to overcome the reciprocity law failure of silver halide emulsions, doping of an Ir complex having a halogen as a ligand is known to be effective. For instance, JP-B-43-4935 (the term "JP-B" as used herein means an "examined Japanese patent publication") mentions that a photographic material having a silver halide emulsion which contains a slight amount of an iridium compound as added during precipitation or ripening of silver halide grains in the emulsion gives an image having an almost constant gradation even when the exposure time is varied in a broad range. However, H. Zwicky (Journal of Photographic Science, Vol. 33, page 201) mentions that exposure of a chlorine ligand-doped high silver chloride emulsion is accompanied by intensification of the formed latent image in the period of from 15 seconds to about 2 hours after the exposure. Such intensification unfavorably causes fluctuation of the sensitivity of the exposed emulsion, depending upon the variation of the time from exposure to processing. Therefore, this emulsion is inconvenient from a practical standpoint.
JP-A-1-105940 corresponding to EP 312994A mentions that a high silver chloride emulsion having a selectively iridium-doped silver bromide-rich region has an excellent reciprocity law characteristic without interfering with the latent image stability for several hours after exposure. However, in accordance with the illustrated technique, overcoming of the reciprocity law failure of a pure silver chloride emulsion is impossible, and variation of the reaction condition in forming the silver bromide-rich region often causes sensitization of latent images formed Because of these reasons, further improvement of the proposed technique is desired.
JP-A-2-20853 corresponding to U.S. Pat. No. 4,945,035 mentions that doping of a high silver chloride emulsion with an Re, Ru or Os six-coordinate complex having at least four cyan ligands is effective for elevation of the sensitivity of the doped emulsion However, as a result of repeated studies by the present inventors on the illustrated technique, it has been clarified that the photographic material having the doped emulsion is often fogged under pressure during development of the material with a developer, though the sensitivity of the emulsion could surely be elevated by the illustrated technique. Because of this drawback, however, the emulsion is practically useless.